4. In what ways can hearing be impaired?

4.1 How is hearing loss defined and classified?

One of the most common forms of hearing impairment is hearing loss

Hearing impairment refers to the complete or partial loss of the ability to hear from one of both ears and can be graded as mild, moderate, severe or profound. When the hearing threshold in the better ear is at or below 25 dB, this will pose very little or no hearing problems. At the other extreme, threshold values in the better ear at or above 81 dB result in the listener being unable to hear and understand even a shouted voice.

Hearing impairment that is caused by a problem in the outer or middle ear can usually be treated. When the impairment is due to problems in the inner ear or the auditory nerve going from the ear to the brain, the hearing loss is usually permanent. Common causes of this type of hearing problem are ageing, excessive exposure to loud sounds and some drugs.

Hearing is usually tested by presenting sounds of different frequencies to the listener and measuring the lowest volume of the sound that the listener can detect. The threshold of hearing is set at 0 dB HL and levels between 0 and 20 are considered to be normal.

Any sign of a shift in this threshold, even within the normal range, could be a sign of impairment so it is important to assess any such change, particularly for children.

Hearing impairment may also arise in people with normal threshold levels of hearing but who cannot process the sound signals properly and therefore find it very difficult to understand speech. Other people have trouble focusing on particular sound frequencies; they cannot tune in to sounds of interest and are distracted by background noise. More...

4.2 How is speech comprehension affected by sound exposure and hearing loss?

The ability to understand speech depends on how loudly a person speaks and on hearing loss, and can be described by mathematical models. A normal-hearing person can understand the words in a sentence if about 30% of the information is present. Listeners can fail to understand speech if the volume of the sound is below the threshold value they can hear, or if there is a background noise that masks the sound signal.

In everyday situations, listeners are exposed to combinations of many different sounds. People with hearing loss at high frequencies have difficulties understanding speech in noisy environments such as a party where there are many different conversations taking place or in large rooms with a lot of echoes such as a church hall. For instance, if a normal-hearing person can communicate at a party at a distance of about one meter, a high-frequency hearing loss of about 40 dB makes it impossible to do so; the listener has to come closer to the speaker and reduce the distance to half a meter.

People with a more significant hearing loss at high frequencies will find it impossible to understand speech in noisy environments unless they get extremely close to the speaker, which may be socially unacceptable. Hearing aids can only partly compensate such loss. Therefore, high-frequency hearing loss, whether aided or not, will cause poorer speech understanding in a noisy environment. More...

4.3 What is tinnitus?

Tinnitus is a condition in which a person hears a ringing, buzzing or hissing sound which is caused by the hearing system and not by any external sources. Tinnitus can either be objective, when the sound can also be detected by an external examiner, or subjective, when no such sound can be detected.

Tinnitus is often associated with hearing impairment, ageing or exposure to loud sounds, and generally involves the part of the nervous system that deals with hearing. Sometimes it is very short-lived and happens only after exposure to loud sounds but it can also be more persistent. Tinnitus that lasts for more than five minutes and not only after loud sounds is called ‘prolonged spontaneous tinnitus’ (PST).

Estimates of the proportion of people with this type of tinnitus vary depending on the method used to collect data. For instance, in a study published in 2007, 17.7% of the people questioned said that they had such tinnitus at some stage in their lives, about 4% had tinnitus most of the time and 0.4% had their quality of life substantially affected by tinnitus. Studies in young people show that people with substantial exposure to loud sounds are more likely to have tinnitus than their unexposed peers, and the higher the exposure the greater the likelihood of developing tinnitus.

Little is known about how subjective tinnitus happens in the body although there are many theories that try to explain it. Some theories involve sensory cells or nerve fibres in the ear which are more active than normal. Other theories focus on the organ of the inner ear that converts sound into electrical impulses (cochlea). When it is damaged, the nervous impulses in the hearing system may become very active and the brain may not be able to suppress them. More...

4.4 How is hearing affected by age?

Hearing ability deteriorates with age in virtually all people and this deterioration accelerates for older people. In other words, with age the hearing threshold is no longer as low as at 18 years old.

In young adults, up to the age of 40, this process is slow and leads to negligible levels of hearing impairment. Some older people are a lot more affected by hearing loss than others. More...